Page 213 - Carbonate Facies in Geologic History

200 Pennsylvanian-Lower Permian Shelf Margin Facies

algal mounds. But analogies with other modern shallow marine lime mud banks,
show how complex the origins of individual mounds may be. Not only is current
orientation probably a controlling factor, but also the absence of currents allow
some modern mounds to form. Where gentle currents meet and are damped out,
mud mounds form, often opposite seaward openings of major lagoons. This is
seen in slack water areas as at Bulkhead Shoal described by Pusey (1964) in the
northern lagoon of British Honduras and Cayo Sucio from the Isla Blanca lagoon
along the northeastern Yucatan coast (Brady, 1971). Furthermore, wind-pro-
duced gyres may also produce mud accumulations in completely closed lagoons.

Variations in Tectonic Settings for Pennsylvanian-Wolfcampian Buildups

The characteristic biota of the buildups described in this Chapter had the capacity
for development in all the major tectonic settings. Composite mounds have devel-
oped large offshore banks in the Midland basin on which perhaps a few hundred
meters of relief developed owing to rapid subsidence and the ability of carbonate
sedimentation to keep pace with it, e.g., Horseshoe Atoll of northwest Texas.
These and shelf marginal buildups appear generally to consist of distinct loaf-
shaped mounds, formed preferentially downslope from paleotopographic crests
(Type I buildup as outlined in Chapter XII, and Wilson, 1974). They tend to offiap
each other downslope if subsidence was not great. No great linearly persistent
barrier reef trends are known along the margins but only irregular chains of
mounds commonly with abundant algal plates and caps of tubular foraminifera
and Tubiphytes. Up on the shelves extensive flank beds of limestone developed
surrounding mound cores. In contrast, mounds down the depositional slope have
few carbonate flank beds and intermound areas are filled with shale or siltstone.
In addition, far across the shelves and removed from the basins, lens-like beds
of the same organic facies as mounds occur but are merely a few feet thick. These
formed in shelf areas distant from influence of terrigenous clastics, usually in the
middle phases of the typical Pennsylvanian cyclothems.

Types of Porosity and Permeability in Pennsylvanian-Wolfcampian Buildups

1. Grainstone accumulations of algal plates, as in the Middle Pensylvanian of
the Paradox basin, afford primary porosity.
2. Most algal plates accumulated in a lime mud matrix, but this also may
develop good porosity through brecciation of the plates and of the partly lithified
matrix. The mud collapsed, fractures formed through partly lithified portions, and
rigid shells and algal plates themselves rotated and fractured.
3. Early solution porosity through internal alteration and decay of the algal
plates offered channelways for subsequent fluid migration. Only the cortex of the
plates were originally calcified, increasing susceptibility to break down. Leaching
of additional void space followed through the action of meteoric water. Infilling of
some voids with internal sediment formed from mud swept through the fabric as
it collapsed. Part of this infilling is the vadose silt of Dunham (1969b).